Metal silicates and other layered clays are known that are arranged in layered structures, stacked in an orderly fashion. Nanocomposites can be obtained by admixing of polymers with the layered clay to break the ordered layering into particles with size less than about 100 nm. The nanocomposites can be made by blending of clay with polymer or blending of clay with monomer followed by in situ polymerization.
There has been considerable interest over the past decade in development of such nanocomposites, for example so-called polymer/layered silicate nanocomposites (PLSNCs), because they can often exhibit remarkably improved mechanical and other properties when compared with the polymer alone or other composites. Some clays commonly used for the preparation of polymer/layered silicate nanocomposites (PLSNCs) belong to the same general family of phyllosilicates. Their crystal structure can consist of layers made up of two silica tetrahedral fused to an edge shared octahedral sheet of either aluminum or magnesium hydroxide. The layer can have a thickness of about 1 nm and the lateral dimensions of these layers can vary from 30 nm to several micrometers and even larger depending on the particular layered silicate. Stacking of the layers can lead to a regular van der Waals gap between the layers called the interlayer or gallery. In order to realize the potential enhancement in properties from the very high aspect ratios of these clay fillers, a polymer matrix can be intercalated between the stacked layers and the individual layers ultimately dispersed throughout the polymer matrix, which can be referred to as exfoliation.
Commonly used layered silicates include montmorillonite (MMT), hectorite and saponite. This type of clay can be characterized by a moderate surface charge, for example, a cation exchange capacity (CEC) of 80 to 120 mequiv/100 g and a layered morphology. These clays can be compatible with hydrophilic polymers such as, for example, poly(ethylene oxide) (PEO) and poly(vinyl alcohol)(PVA).
As described in the recent comprehensive review article by Okamoto “Recent advances in polymer/layered silicate nanocomposites: an overview from science to technology” by M. Okamoto in Materials Science and Technology 22 (2006) 756-778, one of the very few commercially successful example of a PLSNC based on organoclays is that pioneered by Toyota Research wherein the organoclay is first dispersed in caprolactam monomer. The organoclay-caprolactam dispersion is then subjected to condensation polymerization conditions to create a Nylon 6-PLSNC by what is referred to as an in situ process resulting in exfoliation of the nanoclay. As described in the review there are few other examples of successful preparation of PLSNC by an in situ process, yet it is also very difficult to achieve results beyond intercalation by melt extrusion compounding of organoclays into hydrophobic engineering resins. As a result the properties of PLSNCs are seldom very attractive due to the difficulty in achieving substantial amounts of exfoliation.
Unexamined Korean Patent Application 10-2006-0053387, published Dec. 20, 2007, discloses the preparation of PS-PLSNC which can be subsequently used to prepare a PS/PPE (polystyrene/poly(phenylene oxide) blend by extrusion compounding. This patent application discloses the use of a reactive vinylbenzylamine to prepare a reactive organoclay that was then dispersed in styrene monomer and subjected to bulk polymerization. Examples of PS-PLSNC containing up to 5% clay are given which show no signal in XRD and appear exfoliated. When the clay content of the PS-PLSNC is increased to 8% a signal is returned in the XRD scan. The PS-PLSNCs were combined with PPE in 20/80 ratios by melt extrusion and showed an increase in glass transition temperature and storage modulus. With the limitation of no more than 5% clay in the PS—PLSNC and only 20 parts of said PS-PLSNC in the PPO blend, the effective clay concentration in the PPO/PS-PLSNC blend appears limited to 2%.
Thus, it would be desirable to obtain nanocomposites, and articles fabricated from them, that can provide improved properties or advantages, especially by achieving exfoliation of layered nanoparticles in various polymeric compositions.